I assume Obsidian is throwing some of their own money on the project. Either that or a straight up loan. $4M would let you hire at most 15 people for 18 months. A project like PE easily has twice as many involved due to all the stuff that needs to be done beyond development - marketing, project management etc.

However, it's safe to assume they're going to sell quite a bit more than the current pre-order KS sales, so it's not like their total PE income will be $4M. They can probably spend a good $10M on it and still make a healthy profit in the end.